configure.ac: Check for -pthread.
configure.ac: Check for -pthread.

On Tru64, we need to build with -pthread to be allowed to include
pthread.h:

	pthread.h:339:4: error: #error "Please compile the module including pthread.h with -pthread"

Signed-off-by: Lennert Buytenhek <8858498612e3a522354216649776e3032dcb7e5b@wantstofly.org>

buytenh/ivykis,buytenh/ivykis